1. Field of the Invention
The present invention relates to a printed circuit board having a structure in which a device is embedded in a cavity in a wiring board and a fabrication process thereof, in particular, to a printed circuit board having a structure air-tightly isolating a first space on a side of a functional surface and a second space other than the first space and a fabrication process thereof.
2. Description of Related Art
Conventionally, a device, such as a surface acoustic wave (SAW) device, a functional surface of which cannot be directly covered with a sealing member and which should be provided with a cavity on a side of the functional surface, has been air-tightly sealed using weld-sealing or ceramic package so as to be mounted on a surface of a wiring board as a single component.
Since mounting of such a sealed device is limited only to an outermost surface of the wiring board, and further, the device is packaged, an area required for the mounting is larger than that of the size of the original device (bear-chip size). However, the outermost surface has a limited area for mounting parts, and it cannot meet recent demands for miniaturization of the whole wiring board and for higher density mounting.
In view of this, as a method for realizing mounting of more pieces of parts as well as miniaturizing the wiring board, one including forming a hollow in an inner layer of the wiring board and mounting a bear chip being not packaged in the hollow is disclosed in Japanese Patent Application Publication Laid-Open No. Sho 61-112399, for example.
FIG. 10 shows a cross-sectional view of a main portion of a printed circuit board 30. The printed circuit board 30 has a wiring board 40 comprising three substrates 36, 37 and 38 and a device 41 mounted in the wiring board 40 in a state of being a bear chip.
The device 41 is mounted in the wiring layer in a condition that a functional surface 46 thereof faces down in a hollow 47 formed in the substrate 36. The device 41 is electrically connected to an inner wiring formed in the substrate 37 through a plurality of conductive bumps 44. The conductive bumps 44 may include a solder bump, a gold stud bump, and the like. Therefore, a gap g′ as large as a height of the conductive bump 44 (several tens μm, for example) is formed between an inner surface 45 of the hollow 47 and the functional surface 46.
However, such a structure provides insufficient reliability since the conductive bumps 44 are not protected with a resin or the like. The device 41 comprising a semiconductor or a ceramic has a thermal expansion coefficient largely different from that of the wiring board 40 comprising an organic material, which is currently popularly used. Accordingly, the conductive bumps 44 functioning as a junction between the device 41 and the wiring board 40 receive concentrated stress so that cracks are easily occurred in the bumps 44 and the bumps 44 are easily removed from the device 41 and/or the wiring board 40. Particularly, since the conductive bumps 44 used in a case of mounting the bear-chip device 41 facing down are small in size and short in height, it is required to make up for the strength of the bump junctions.
On the other hand, Japanese Patent Application Publication No. Hei 8-274575 discloses a structure in which all spaces, other than a part which the functional surface 46 faces toward within the gap g′, is sealed with a resin 61 in the hollow 47, as shown in FIG. 11, for example. This structure ensures the reliability of the conductive bumps 44 by dispersing the stress concentrated on the bumps 44 over the resin 61.
However, the structure of FIG. 11 possibly brings a problem of the stress affecting on the device 41 through the resin 61. The stress includes a thermal stress. In general terms, a resin material shows its large thermal shrinkage. The stress caused by the thermal shrinkage of the resin 61 acts on the device 41 which the resin 61 contacts with. Mechanical stress may easily act on the device 41 as well as the thermal stress. A case of employing a SAW device for the device 41 will be considered herein. The SAW device is frequently incorporated in a cellular phone as a filter device. In the cellular phone, the wiring board 40 easily deforms due to operation on buttons or keys. The stress caused by the deformation of the wiring board 40 acts on the device 11 indirectly contacting with the wiring board 40 via the resin 61 disposed therebetween. In particular, since materials including LiTaO3 used for SAW devices are weaker than a silicon semiconductor and are easily broken, it is necessary to avoid stress acting on the device 41 employing such materials.